The present invention relates to a conductivity modulating MOSFET, wherein base current of bipolar junction transistors (BJT) is supplied by channel current of the MOSFET.
The conductivity-modulating MOSFET of the invention is also called an insulated gate-bipolar transistor (hereinafter abbreviated as IGBT). In the present power electronics field, the most attracted power devices are power MOSFET and the IGBT. The reason for this is that both provide high-speed switching performance and low-drive power consumption, which result in compactness and improved performance required by power electronics products. However, in the unlimited expansion of use and improved performance of power electronics products, the use of a larger current, higher voltage, and switching performance at a higher speed are required. In the BJT and IGBT module products used in inverter circuits for motor control, the module products with a withstand voltage of 1200 V and a current capacity of 400 to 800A have already been made available. The IGBT is by far superior to the BJT in terms of its high-speed switching performance. However, because of the high switching speed, in an inductance load generally used for switching a large power, IGBT generates an excessive spike voltage due to its large di/dt and when the voltage exceeds the breakdown voltage of a device, the device may be destroyed. Even if the voltage does not exceed the breakdown voltage, the electric field strength increased by the simultaneous application of high voltage and large current may exceed the breakdown field and generate avalanche multiplication, which results in destruction of a device. Particularly, under abnormal conditions, such as a load by short circuit, the power device is forcibly turned off in about 10 micro sec after an abnormal condition has been detected. At this time a very large short circuit current as well as a high withstand voltage are applied as shown in FIG. 2, and a further higher spike voltage is applied as shown by B because of di/dt by the turn-off of the device shown by A, which results in a breakdown or destruction due to the aforementioned mechanism.
In order to protect the device in the abnormal condition, it is very difficult to absorb energy in excess of the breakdown voltage in an IGBT of a high withstand voltage because of uneven electric field. Therefore, it is difficult to manufacture an IGBT with a high withstand voltage and a high avalanche withstand capacity with high yield and reliability.
Accordingly, it was proposed that as shown in FIG. 3, a high withstand voltage diode (32) with high avalanche withstand capacity of a withstand voltage BV.sub.R, which is slightly lower than the breakdown voltage BV.sub.CES of an IGBT (31), and a low withstand voltage diode (33) with a withstand voltage of about 20 to 30 V, which is slightly higher than the voltage between the gate and the emitter in the IGBT (31), are externally attached between the gate and the collector. In case overvoltage is generated in an abnormal condition, it exceeds the breakdown voltage BV.sub.R of the diode (32), which is lower than the overvoltage BV.sub.CES of the IGBT (31). Therefore, the current flows to a gate electrode (34) of the IGBT to charge the gate-to-emitter capacity of the IGBT (31). When its threshold voltage is exceeded, the IGBT (31) turns on to absorb the overvoltage energy evenly into the chip. As a result, it can withstand the larger energy.
However, the method of connecting the diodes (32) and (33) by means of an external installation as shown in FIG. 3 causes the following drawbacks.
(a) Diodes with a withstand voltage slightly lower than that of the IGBT (31) must be selected individually, which is not practical.
(b) In case of an external installation, an inductance content is added thereto because of long wiring.
As a practical method for obviating the deficiency of (a), diodes which have withstand voltages considerably lower than the minimum withstand voltage of the IGBT are selected and connected. However, considering the wide range of the withstand voltage in the diodes, a withstand voltage which is significantly lower than that of the IGBT has to be admitted. This requires that the withstand voltage of the IGBT be shifted preliminarily to the high side. However, this choice aggravates the relation between the on-voltage of IGBT and the switching time.
For instance, assuming that the IGBT has a withstand voltage of 1,250 V, diodes having a withstand voltage within a range of 1,050 V to 1,200 V are used while considering the variance of the withstand voltage. In this case, unless the voltage applied to switching at every cycle is lower than a maximum of 1,050 V, there is a good chance of a breakdown as a result of increased switching loss. This in turn means that even the IGBT with a withstand voltage of 1,250 V can only guarantee a spike voltage of 1,050 V or less, which is 200 V lower than the withstand voltage.
Accordingly, an object of the present invention is to obviate the above problems and to provide an IGBT which allows the withstand voltage of each and every transistor to be set at an approximately definite value of 50 V or 100 V less than the withstand voltage of protection diodes without selection.